Many superconducting devices such as magnetic resonance imaging (MRI) systems use an inventory of liquid cryogen (i.e. helium) for providing the continuous refrigeration necessary to maintain a temperature well suited for superconduction. Typically, a cryostat or vacuum jacketed reservoir of the liquid cryogen is used to cool the device to achieve superconductivity. As the device is used, heat is generated and transferred to the inventory of liquid cryogen. When the liquid cryogen absorbs sufficient heat, it boils resulting in a large portion of the cryogen becoming gaseous. In the case of mobile magnetic resonance imaging (MRI) systems, additional quantities of cryogen are boiled off each time the system is magnetized and demagnetized. It is necessary to demagnetize such devices for every road trip.
In order to maintain and replenish the inventory of liquid cryogen so as to compensate for the boiled-off cryogen, a continuous supply of gaseous cryogen must be provided. This gaseous cryogen must be liquified and introduced into the liquid inventory. To help reduce the amount of cryogen that must be replaced into the system, a means of recondensing the boiled-off cryogen back into the liquid inventory can be used. One means of recondensing the boiled-off cryogen is to collect the venting cryogen gas and direct it to a refrigeration apparatus (cryogenic recondenser) located above the cryostat which recondenses the cryogen. Once the cryogen is recondensed, it is then reintroduced back into the cryostat. A variation on this approach is presented in U.S. Pat. Nos. 4,766,741 and 4,796,433, a transfer line which carries a liquid cryogen (i.e. helium) from a closed cycle refrigeration system is inserted into the cryostat. The gaseous cryogen in the cryostat is cooled by heat exchange with the liquid cryogen in the transfer line to such an extent that it recondenses.